In conventional production techniques in the production of oil, the wells are operated so as to change the reservoir saturations as little as possible. That is, the wells are produced at moderate rates with minimum reservoir pressure drop in order to allow the gas in solution in the oil to be produced along with the oil so as to retain the gas energy to "lift" the oil to the surface. In doing this, a primary concern is to preclude the gas from separating from the oil in the reservoir. Gas separated from its oil in the reservoir will migrate due to its lower density (compared to oil) to a high structural location and become unavailable for lifting the oil to the surface. Separation of solution gas from its oil in the reservoir normally is minimized also for the reason that excessive gas would be produced along with oil usually resulting in a decrease in the allowable oil production by regulatory bodies. And importantly, the loss of gas from oil impedes the flow of oil to the well.
In a geo-pressured oil reservoir having a geo-pressured water drive, the unrecovered, bypassed or "residual" oil remaining in a "watered-out" reservoir, that is, where oil production ceases due to the invasion of water, will exist at essentially the original pressure of the reservoir and will contain essentially its original quantity of solution gas.
It is known that the residual hydrocarbon saturation remaining after "watering out" of an oil reservoir is some fixed fraction of the pore volume of the reservoir. The hydrocarbon occupying this fraction of the reservoir pore volume is liquid (crude oil) or gaseous (natural gas), and can exist as free oil or gas or dissolved gas. Gas will be dissolved in both the crude oil and in the water coexisting in the pore space. The crude oil is barely soluble in reservoir waters and therefore, will exist as free oil.
The coast of the United States bordering the Gulf of Mexico has numerous geo-pressured reservoirs or aquifers in which "watered-out" oil reservoirs exist.
Our prior copending application Ser. No. 689,622, now U.S. Pat. No. 4,042,034, disclosed the concept of increasing the recovery of natural gas from geo-pressured aquifers by producing water from wells at a high production rate to reduce formation pressure significantly whereby an increased amount of natural gas could be released and recovered. The amount of gas in solution with water is generally only about 30 to 50 scf per barrel of water. It is much more important to recover the hydrocarbons from a depleted oil well than a gas well as the recovery can result in a greater unit volume of energy sources because the oil is more valuable than gas, and also the amount of gas in solution with oil may be 500 scf per barrel of oil or more.